The present invention relates to chromia spray powders and to a process for making such powders. Such powders are useful in the production of thermally sprayed coatings on substrates.
It is well known that placing a spray coating of chromia on a substrate, which is usually a metal, confers a very significant improvement in the hardness and wear resistance of the surface. For this reason embossing rolls or rotogravure rolls are often given a chomia coating. More generally chromia coatings are used on many parts that are subjected to wear such as pump bodies, shafts, rolls and printing rolls. These parts can be used as sprayed or may go through subsequent finishing processing such as grinding, lapping or polishing. Printing rolls are usually further processed by laser engraving to make a pattern of cells that are useful for carrying ink. The coatings can be applied by a number of techniques but the most frequently used is based on the use of a thermal spray process in which the ceramic particles are injected into a plasma jet directed towards the substrate. The heat of the plasma jet melts the ceramic particles and causes them, upon impact with the substrate, to form a ceramic layer with a high degree of uniformity and integrity that is capable of protecting the substrate on which it is coated, giving the substrate the superficial hardness and wear characteristics of the ceramic with which it is coated.
There is however a problem because, upon thermal spraying a chromia powder on to a substrate, some of the chromia reacts with oxygen and impurities in the chromia to yield very toxic hexavalent chromium compounds. The hexavalent chromium compounds are formed at elevated temperatures such as are used in thermal spraying ceramic powders and seem to be formed in the flame and especially in the outer parts of the flame. Material in this outer part is not heated to quite the same temperature and does not adhere to the surface sprayed to the same degree. The result is an undesirably high level of hexavalent chromia in the powder not adhered but remaining to be recycled or disposed of, thus presenting a significant environmental problem.
In an experiment to determine the extent of this effect, the amount of hexavalent chrome in a thermal spray chromia powder was measured at 39 ppm and a coating of this powder thermally sprayed on to a substrate showed a concentration of 10 ppm. However when the overspray was sampled the amount of hexavalent chrome varied between 470 and 8800 ppm. Chromia (Cr2O3) converts to the hexavalent state in the presence of oxygen at temperatures over 1000° C. but reverts to chromia on cooling. However in the presence of alkali metal or alkaline earth metal impurities, or certain other impurities known in the art, the chromium forms complex compounds which stabilize the chromium in the hexavalent state.
There is therefore a urgent need for a new coating material that does not have the tendency of present chromia-based wear-resistant coating powders to oxidize to the hexavalent state when used in a thermal spray process and does not have the significant loss of hardness and wear resistance that characterize the conventional chromia coatings.